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Shaftless Screw Conveyors: a Perfect Solution for Handling Bulk Solids

April 12, 2012
The original Archimedes screw

The screw conveyor has been around since 267 BC when Archimedes came up with the brilliant idea to use a spiral incline plane to pump water for irrigation purposes. Figure 1 shows Archimedes’ original design. Since then, the screw conveyor has been used in thousands of industrial applications, every day, all over the world, for conveying and metering bulk materials.

While the basic concept of the screw conveyor has not changed, there have been many improvements in the design due to the needs of end users and the demands of the applications. The focus of this article is on shaftless screw conveyors. The shaftless screw conveyor is a fairly recent development in screw conveyor design and is growing in popularity because it is the perfect solution for many bulk solids handling applications.

Origin of the Shaftless Screw Conveyor
The shaftless screw conveyor was developed in the early 1940s in Europe as a means to convey dewatered biosolids for the wastewater treatment industry. Biosolids can be very sticky and sluggish, depending on solids content. Most biosolids consist of 20 to 30 percent solids with the balance being moisture contained in the solids. Figure 2 shows a typical shaftless screw conveyor for biosolids. The shaftless screw conveyor is the solution for conveying many difficult bulk solids. In fact, the shaftless conveyor was so successful in the wastewater treatment industry, that its use has been promoted in many other industries such as chemical, food and minerals processing.

Comparison of Shafted to Shaftless Conveyors
The shaftless screw conveyor is quite similar to a conventional shafted screw conveyor in design and construction. Both types of screw conveyors consist of a spiral or screw, troughs, trough ends, bearings, inlets, discharges, covers, and a drive unit. The unique difference between a shaftless and a conventional shafted screw conveyor is the spiral, or screw. A conventional shafted screw conveyor has a center pipe to support the screw flights, and the pipe is supported on each end by bearings. Typically, there is a half-inch gap between the outside diameter of the screw and the inside diameter of the trough and therefore, the screw never touches the bottom of the trough. In contrast, a shaftless conveyor is designed to be supported by a liner that conforms to the radius of the trough. The weight of the shaftless spiral is distributed over the full length of the spiral on the liner. The drive is connected to one end of the spiral by means of a flanged and bolted drive shaft and spiral end plate, while the tail end of the spiral is not connected and is allowed to rotate freely. Figure 3 shows a comparison between the two conveyor types.

Applications for Shaftless Screw Conveyors
Most shaftless conveyors are designed to be used to handle bulk solids that have high moisture content. The handling of bulk solids discharged from centrifuges, filter presses, or mixers fall into this category and represent the proper application for shaftless screw conveyors. These bulk solids are typically sticky and difficult to convey in conventional shafted screw conveyors. Sticky bulk solids tend to adhere to the center pipe of a shafted screw conveyor where the flight meets the pipe. Shaftless screw conveyors totally eliminate this problem because there is no center pipe and only the spiral is used to convey the bulk solid. If your process consists of bulk solids with high moisture content, then the shaftless screw conveyor is a perfect solution.

Component Selection
When selecting the proper shaftless screw conveyor for your application, many factors must be considered. The spiral consists of a rectangular bar made of high-strength steel that is manufactured or rolled to the required diameter. Each shaftless screw conveyor manufacturer has its own unique alloy for the shaftless spiral. Some manufacturers have several spiral types to choose from, depending on the application. The strength and hardness of the spiral is very important to the long-term success of the installation. For example, if you are conveying an abrasive bulk solid, you need a spiral that is extremely hard in order to reduce the amount of abrasion on the shaftless spiral. Top manufacturers offer spirals with a hardness of up to 350-BHN. Since there is no center pipe to transmit torque, the shaftless spiral must be designed to handle the full-torque load of the drive unit. Make sure the shaftless screw conveyor manufacturer can back up their design with calculations for the torsional rating of the spiral.

The trough liner material is also important to the long-term success of the installation. Typical trough liner material is ultra-high molecular weight polyethylene or UHMW. UHMW is available in many different grades from virgin to oil-impregnated. Another popular trough liner material is Xylethon. Both of these materials are tough and have a low coefficient of sliding friction. Since the spiral is bearing down on the liner and rotating, it helps to have a tough, slick liner material. The liner material is typically the wear item and is easier and less expensive to replace than the shaftless spiral. The liners can be removed and replaced in minimum time and with few tools.

Because the spiral is not connected on the tail end, a hold-down device should be located either on top or on the sides of the trough to make sure the shaftless spiral does not rise up if a large chunk of product gets under the spiral. The hold-down is designed for easy removal to allow access for maintenance or for liner replacement. Each shaftless screw conveyor manufacturer has its own style of hold-down.

Versatility of Design
The shaftless design can be used as either a screw conveyor or a screw feeder. A screw conveyor is always control fed and transfers bulk solids from one location to another. A screw feeder is located under a hopper, bin, or silo, and is used to meter the bulk solid at a controlled rate to the downstream process. The speed of a shaftless screw feeder can be changed using a variable frequency drive (VFD). A good application for a shaftless screw feeder is for metering a moist cake material from a hopper to a direct or indirect dryer.
Shaftless conveyors can be designed to operate horizontally, at varying degrees of incline, and even vertically. Consult your favorite shaftless screw conveyor manufacturer to determine the best design for your application.

Conclusion
The screw conveyor has certainly changed since the days of Archimedes. However, it has remained a popular solution to many bulk solids handling applications. It is a cost-effective, versatile, and rugged device that can be designed for conveying or metering almost any bulk solid. The shaftless screw conveyor is an improvement to the conventional shafted screw conveyor design and could be the ideal solution for your bulk solids handling application.

Bill Mecke, P.E. is President of KWS Manufacturing Company, Ltd, one of the largest screw conveyor manufacturers in North America. He has more than 20 years of experience working directly with customers to solve their material handling needs. Mecke earned his Bachelor of Science degree in Mechanical Engineering from Texas A&M University. He is also a registered Professional Engineer in the State of Texas. For more information, call 800-543-6558 or visit www.kwsmfg.com.

KWS Manufacturing Company, Ltd.